U.S. patent application number 15/327012 was filed with the patent office on 2017-06-08 for material for a thermoelectric element and method for producing a material for a thermoelectric element.
The applicant listed for this patent is EPCOS AG. Invention is credited to Hermann Gruenbichler, Manfred Schweinzger, Yongli Wang.
Application Number | 20170158563 15/327012 |
Document ID | / |
Family ID | 53724316 |
Filed Date | 2017-06-08 |
United States Patent
Application |
20170158563 |
Kind Code |
A1 |
Gruenbichler; Hermann ; et
al. |
June 8, 2017 |
Material for a Thermoelectric Element and Method for Producing a
Material for a Thermoelectric Element
Abstract
A material for a thermoelectric element and a method for
producing a material for a thermoelectric element are disclosed. In
an embodiment the thermoelectric element includes a material
comprising calcium manganese oxide that is partially doped with Fe
atoms in positions of Mn atoms.
Inventors: |
Gruenbichler; Hermann; (St.
Josef, AT) ; Wang; Yongli; (Frauental, AT) ;
Schweinzger; Manfred; (Schwanberg, AT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EPCOS AG |
Muenchen |
|
DE |
|
|
Family ID: |
53724316 |
Appl. No.: |
15/327012 |
Filed: |
July 7, 2015 |
PCT Filed: |
July 7, 2015 |
PCT NO: |
PCT/EP2015/065470 |
371 Date: |
January 17, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 2235/3227 20130101;
C04B 2235/5445 20130101; C04B 2235/3294 20130101; C04B 2235/9607
20130101; C04B 2235/6567 20130101; H01L 35/34 20130101; C04B
2235/5463 20130101; H01L 35/22 20130101; C04B 2235/3274 20130101;
C04B 2235/3201 20130101; C04B 2235/80 20130101; C04B 2235/3268
20130101; C04B 2235/3284 20130101; C04B 2235/3298 20130101; C04B
2235/3275 20130101; C04B 2235/768 20130101; C04B 2235/79 20130101;
C04B 2235/6565 20130101; C04B 2235/656 20130101; C04B 2235/661
20130101; C04B 2235/549 20130101; C04B 2235/3206 20130101; C04B
35/016 20130101; C04B 2235/3213 20130101; C04B 35/6261 20130101;
C04B 2235/3263 20130101; C04B 2235/786 20130101; C04B 35/62695
20130101; C04B 2235/3208 20130101; C04B 2235/3224 20130101; C04B
2235/3272 20130101; C04B 2235/3215 20130101; C04B 35/62675
20130101; C04B 2235/77 20130101; C04B 35/64 20130101 |
International
Class: |
C04B 35/01 20060101
C04B035/01; H01L 35/22 20060101 H01L035/22; H01L 35/34 20060101
H01L035/34; C04B 35/64 20060101 C04B035/64 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 17, 2014 |
DE |
10 2014 110 065.4 |
Claims
1-15. (canceled)
16. A thermoelectric element comprising: a material comprising
calcium manganese oxide that is partially doped with Fe atoms in
positions of Mn atoms.
17. The thermoelectric element according to claim 16, wherein a
doping with Fe atoms provides a content of z.ltoreq.20% at the
positions of Mn atoms.
18. The thermoelectric element according to claim 16, wherein the
material is further doped with an element that provides electrons
for electrical conductivity in positions of Ca.sup.2+ atoms.
19. The thermoelectric element according to claim 18, wherein the
element is selected from the group consisting of rare earth metals,
Sb.sup.3+, and Bi.sup.3+.
20. The thermoelectric element according to claim 18, wherein a
doping with the element provides a content of 0<y.ltoreq.0.5 at
the positions of Ca atoms.
21. The thermoelectric element according to claim 16, wherein the
material is further doped with a divalent element in positions of
Ca.sup.2+ atoms.
22. The thermoelectric element according to claim 21, wherein the
divalent element is selected from a group consisting of Mg.sup.2+,
Sr.sup.2+, Ba.sup.2+, Zn.sup.2+, Pb.sup.2+, Cd.sup.2+ and
Hg.sup.2+.
23. The thermoelectric element according to claim 21, wherein a
doping with the divalent element provides a content of
0<x.ltoreq.0.5 at the positions of Ca atoms.
24. The thermoelectric element according to claim 23, wherein the
doping with the divalent element provides a content of
x.gtoreq.0.05.
25. The thermoelectric element according to claim 16, wherein the
material is represented by the general formula
Ca.sub.1-x-yISO.sub.xDON.sub.yMn.sub.1-zFe.sub.zO.sub.n, wherein
ISO denotes a divalent element that can replace Ca.sup.2+ in a
crystal lattice, wherein DON denotes an element that can replace
Ca.sup.2+ in the crystal lattice and provides electrons for
electrical conductivity, and wherein 0.ltoreq.x.ltoreq.0.5;
0<y.ltoreq.0.5; 0.0001.ltoreq.z<0.2; n.gtoreq.2.
26. The thermoelectric element according to claim 16, further
comprising a second material based on the composition
(Ca.sub.3-xNa.sub.x)Co.sub.4O.sub.9-.delta., wherein
0.1.ltoreq.x.ltoreq.2.9 and 0<.delta..ltoreq.2.
27. A method for producing a material for a thermoelectric element,
the method comprising: firing a material, wherein, for a maximum
temperature T.sub.max, T.sub.max.gtoreq.T.sub.S-75.degree. C. is
true, wherein T.sub.S denotes a melting temperature of the
material, and wherein a maintenance time of at least 30 minutes is
observed on cooling at a preset temperature.
28. The method according to claim 27, wherein the temperature
during the maintenance time is in a range of 700.degree. C. to
800.degree. C.
29. The method according to claim 27, wherein the maximum
temperature is greater than or equal to T.sub.S-75.degree. C. for
at least 10 hours.
30. The method according to claim 27, wherein a cooling rate of
less than or equal to 1.degree. C./min is used in cooling.
Description
[0001] This patent application is a national phase filing under
section 371 of PCT/EP2015/065470, filed Jul. 7, 2015, which claims
the priority of German patent application 10 2014 110 065.4, filed
Jul. 17, 2014, each of which is incorporated herein by reference in
its entirety.
TECHNICAL FIELD
[0002] A material for a thermoelectric element and a method for
producing a material for a thermoelectric element are provided. For
example, the material is an electron conductor based on a complex
metal oxide, particularly a ceramic.
BACKGROUND
[0003] The increase in global energy consumption is causing
increased production of waste heat, which is often not used at all
or used only to an insufficient extent. For example, even in modern
combustion engines in automobiles, a large portion of the energy is
lost via the exhaust as waste heat. Thermoelectric conversion
offers an attractive possibility for increasing general efficiency
in energy production and can play a role in reducing CO.sub.2
production. Use of a thermoelectric element does not require any
moving parts, which are subject to wear. Moreover, there is no
accumulation of waste products such as carbon dioxide that have
adverse climatic effects.
[0004] The dimensionless figure of merit, ZT, can be used to
describe the thermoelectric efficiency of a material. This value is
derived from the following equation:
ZT = ( .sigma..alpha. 2 ) T .kappa. ( 1 ) ##EQU00001##
where .sigma. denotes electrical conductivity, .alpha. the Seebeck
coefficient ("thermopower"), T temperature, and .kappa. thermal
conductivity.
[0005] The publication DE 11 2008 002 499 T5 discloses a method for
producing a complex metal oxide that can be used as a
thermoelectric conversion material.
SUMMARY OF THE INVENTION
[0006] Embodiments of the invention provide an improved material
for a thermoelectric element and an improved method for producing a
material for a thermoelectric element.
[0007] According to a first aspect of the present invention, a
material for a thermoelectric element is provided. The material
comprises calcium manganese oxide, preferably of the general
formula CaMnO.sub.3. The calcium manganese oxide is partially doped
with Fe atoms in the positions of Mn atoms.
[0008] The material is preferably in the form of a perovskite
crystal structure represented by the general formula ABO.sub.3,
where A denotes the A positions and B the B positions of the
perovskite lattice. The A positions are primarily occupied with
Ca.sup.2+ atoms, and the B positions are primarily occupied by
Mn.sup.4+ atoms. In doping with Fe atoms, portions of the B
positions are occupied by Fe.sup.4+ atoms. This corresponds to
"isovalent" doping without a donor effect.
[0009] It has been found that the thermopower of the material can
be improved by doping with iron. According to equation (1),
therefore, the figure of merit of the material can be increased. In
addition, a reduction in the thermal conductivity of the material
is to be expected in doping with iron, which contributes toward
further improvement of the figure of merit.
[0010] In an embodiment, doping with Fe atoms is provided with a
content z, where z.ltoreq.20%. This means that up to 20% of the Mn
positions in the lattice, in particular the B positions in the
perovskite lattice, are occupied by Fe.sup.4+ atoms. In particular,
the amount can be in the range of 0.01% to 20%. In an embodiment,
z.ltoreq.5%, and in particular, 0.01%.ltoreq.z<5% is true.
[0011] The material is preferably of the "n-type". In an "n-type"
material, electrons are present as charge carriers. In a "p-type"
material, holes are present as charge carriers.
[0012] In an embodiment, Ca atoms in the material are partially
replaced by other atoms in order to further improve the properties
of the material. In particular, doping is provided in the A
position of the Perovskite lattice.
[0013] In an embodiment, the material is partially doped with an
element that replaces Ca.sup.2+ in the crystal lattice and provides
electrons for electrical conductivity. This makes it possible to
increase the number of charge carriers. For example, the element is
selected from a group consisting of the rare earth metals,
Sb.sup.3+, and Bi.sup.3+. The group is preferably composed of
Y.sup.3+, Sc.sup.3+, La.sup.3+, Nd.sup.3+, Gd.sup.3+, Dy.sup.3+,
Yb.sup.3+, Ce.sup.4+, Sb.sup.3+ and Bi.sup.3+.
[0014] For example, doping with the element that can replace
Ca.sup.2+ in the crystal lattice and provides electrons for
electrical conductivity can be provided with a content y, where
0%<y.ltoreq.50%. This means that up to 50% of the positions of
Ca atoms are occupied by this element. y is preferably .gtoreq.1%.
y is preferably .ltoreq.10%.
[0015] In an embodiment, the material is partially doped with a
divalent element in the positions of Ca.sup.2+ atoms. This
therefore constitutes isovalent doping. For example, the divalent
element is selected from a group consisting of Mg.sup.2+,
Sr.sup.2+, Ba.sup.2+, Zn.sup.2+, Pb.sup.2+, Cd.sup.2+, and
Hg.sup.2+. Sr.sup.2+ is preferably used.
[0016] For example, doping with the divalent element is provided
with a content x, where 0%<x.ltoreq.50% of the positions of the
Ca atoms is true. x is preferably .gtoreq.5%. x is preferably
.ltoreq.20%.
[0017] In an embodiment, calcium manganese oxide is represented by
the general formula CaMnO.sub.n, where n denotes the formula units
of oxygen. In particular, n.gtoreq.2 is true. Preferably, n.about.3
or n=3 is true. The manganese contained in the compound can have
different valencies. In particular, it is possible for a portion of
the manganese to be reduced from Mn.sup.4+ to Mn.sup.3+. In order
to ensure charge neutrality within the compound, some oxygen may be
removed so that n is formally less than 3.
[0018] In an embodiment, the material is represented by the
following general formula:
Ca.sub.1-x-yISO.sub.xDON.sub.yMn.sub.1-zFe.sub.zO.sub.n
[0019] where
[0020] Ca is the chemical symbol for calcium,
[0021] ISO is a divalent element that can replace Ca.sup.2+ in the
crystal lattice,
[0022] DON is an element that can replace Ca.sup.2+ in the crystal
lattice and provides electrons for electrical conductivity,
[0023] Mn is the chemical symbol for manganese,
[0024] Fe is the chemical symbol for iron, and
[0025] O is the chemical symbol for oxygen,
[0026] where x, y, and z denote the contents of the respective
elements and n denotes the formula units of oxygen.
[0027] For example, x, y, z, and n can be selected as described
above.
[0028] In an embodiment, x, y, z, and n are in the following
ranges:
[0029] Content of ISO: 0.ltoreq.x.ltoreq.0.5, and particularly
0.05.ltoreq.x.ltoreq.0.20
[0030] Content of DON: 0.ltoreq.y.ltoreq.0.5, and particularly
0.01<y.ltoreq.0.10
[0031] Content of Fe: 0.0001.ltoreq.z<0.2
[0032] Formula units of oxygen: n.gtoreq.2, and particularly
n.about.3.
[0033] The material preferably contains few or no elements that are
costly or toxic. In particular, the material is free of selenium
and tellurium. The material can therefore be produced in a
relatively cost-effective manner.
[0034] Moreover, the invention provides a thermoelectric element
composed of the above-described material. For example, the
thermoelectric element is used as a generator.
[0035] For example, two conductors comprising different materials
can be electrically connected to one another in the thermoelectric
element. In particular, one conductor may comprise a material of
the n-type and the other conductor a material of the p-type. The
doped calcium manganese oxide described here is preferably used as
the material of the n-type. For example, the materials can be
configured as rod- or disk-shaped components.
[0036] In an embodiment, the thermoelectric element additionally
comprises a material of the p-type. Sodium cobaltate is
particularly suitable for this purpose. For example, the material
is based on a composition represented by the formula
(Ca.sub.3-xNa.sub.x)Co.sub.4O.sub.9-.delta., where
0.1.ltoreq.x.ltoreq.2.9 and 0<.delta..ltoreq.2, and preferably
0.3.ltoreq.x.ltoreq.2.7 and 0<.delta..ltoreq.1 is true. It has
been found that such a material shows high thermopower and high
conductivity.
[0037] In an embodiment, a plurality of thermoelectric elements is
interconnected to form a module. At least one thermoelectric
element comprises the above-described material based on calcium
manganese oxide.
[0038] The material in preferably mass-produced in a simple manner
using the methods of technical ceramics. In particular, there is no
need for cost-intensive processes such as spark plasma sintering or
firing in special gas mixtures such as Ar/H.sub.2.
[0039] According to a further aspect of the present invention, a
method for producing a material for a thermoelectric element is
provided. In particular, the above-described material can be
produced by this method. All of the properties disclosed with
respect to the material are also correspondingly disclosed with
respect to the method, and vice versa, even if the respective
property is not expressly mentioned in the context of the
respective aspect. However, the method can also be used for
producing another material for a thermoelectric element. In
particular, this can be a material based on calcium manganese oxide
that is not doped with Fe atoms.
[0040] The method comprises a firing process, wherein the maximum
temperature in the firing process is just above the melting point
of the material. For example, the maximum temperature T.sub.max is
.gtoreq.T.sub.S-75.degree. C., where T.sub.S denotes the melting
temperature of the material. The maximum temperature should be
selected in such a way that no melting of the material occurs. The
maximum temperature should preferably be at least 10.degree. C.
below the melting temperature.
[0041] The high firing temperature allows favorable growth of
polycrystals to be achieved. In particular, the high firing
temperature makes it possible to reduce the number of grain
boundaries per unit length. In this manner, a material having high
electrical conductivity can be produced.
[0042] In an embodiment, the temperature is maintained in the
aforementioned range for several hours, for example, at least 10
hours.
[0043] Moreover, sintering is provided in an atmosphere containing
sufficient oxygen. For example, sintering is provided in an air
atmosphere or an oxygen-enriched atmosphere.
[0044] Moreover, the method is also characterized by a slow cooling
rate. In particular, a cooling rate is used of less than or equal
to 2.degree. C./min, and preferably less than or equal to 1.degree.
C./min. Such a cooling rate is used in particular in cooling from
1000.degree. C. to 600.degree. C. The slow cooling rate protects
the material as it goes through phase transitions and therefore
makes it possible to produce a ceramic with few or no cracks.
[0045] In cooling, moreover, preferably in the range of
1000.degree. C. auf 600.degree. C., it is advantageous to have a
maintenance time of at least 30 minutes, and preferably at least
one hour. For example, the temperature during the maintenance time
is in the range of 700.degree. C. to 800.degree. C. e.g.,
750.degree. C. This additional maintenance time allows re-oxidation
of Mn.sup.3+ to Mn.sup.4+ to be carried out as completely as
possible and improves thermoelectric properties such as thermopower
and electrical conductivity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] In the following, the subject matter described here is
explained in further detail based on working examples depicted
schematically and not to scale.
[0047] The figures show the following:
[0048] FIG. 1 is a diffractogram of a material for a thermoelectric
element,
[0049] FIG. 2 is a diagram of electrical conductivity as a function
of maximum firing temperature for two materials,
[0050] FIG. 3 is a micrograph of a material,
[0051] FIG. 4 is a diagram of electrical conductivity as a function
of temperature for a material,
[0052] FIG. 5 is a diagram of the Seebeck coefficient as a function
of temperature for the material of FIG. 4,
[0053] FIG. 6 is a diagram of thermal conductivity as a function of
temperature for the material of FIG. 4,
[0054] FIG. 7 is a diagram of figure of merit as a function of
temperature for the material of FIG. 4,
[0055] FIG. 8 is a diagram of thermal conductivity as a function of
temperature for two further materials,
[0056] FIG. 9 is a diffractogram of two materials,
[0057] FIG. 10 is a diagram of sintering density as a function of
Fe content in a material,
[0058] FIG. 11 is a diagram of the Seebeck coefficient as a
function of Fe content in the material of FIG. 10,
[0059] FIG. 12 is a diagram of sintering density as a function of
Fe content in two materials,
[0060] FIG. 13 is a diagram of the Seebeck coefficient as a
function of Fe content in the two materials of FIG. 12, and
[0061] FIG. 14 is a working example of a thermoelectric generator
having a plurality of thermoelectric elements.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
Method for Producing the Material
Example: Preparation of
Ca.sub.0.85Sr.sub.0.10Dy.sub.0.05Mn.sub.0.975Fe.sub.0.025O.sub.3
[0062] A method is first described for producing a material for a
thermoelectric element.
[0063] For example, the method is used to produce a material of the
composition
Ca.sub.0.85Sr.sub.0.10Dy.sub.0.05Mn.sub.0.975Fe.sub.0.025O.sub.3.
However, the method is not limited to this material, but is also
suitable for producing other materials for thermoelectric
elements.
[0064] For example, the material, preferably a complex metal oxide,
can be produced by means of the so-called "mixed oxide" process.
However, it is also possible to use other production methods, such
as wet-chemical routes or mechanical alloying.
[0065] Stoichiometric amounts of CaCO.sub.3, SrCO.sub.3,
Mn.sub.3O.sub.4, Fe.sub.2O.sub.3, and Dy.sub.2O.sub.3 are weighed
in and wet-ground (using deionized water). A microfine grain size
is achieved using suitable fine milling technology such as a
planetary mill or an agitator bead mill. The grain size
distribution is preferably d(0.5)<1 .mu.m and d(0.9)<1.5
.mu.m. This makes it possible to achieve sufficient reactivity in
the subsequent calcining process. The milled suspension is dried
and sifted.
[0066] Calcination, in which a solid-state reaction takes place to
form a complex metal oxide, is carried out, for example, at
1100.degree. C. in an air atmosphere for several hours. In this
reaction, a largely single-phase material is preferably obtained.
Small amounts of unreacted raw materials from second phases can
further react in subsequent firing to form a complex metal
oxide.
[0067] FIG. 1 shows an x-ray diffractogram (XRD) of the working
example. The measured radiation intensities I are plotted against
the angle of the radiation source, sample, and detector (2.theta.
angle). A comparison with the values reported in the literature for
CaMnO.sub.3 shows that incorporation of Fe atoms has taken place
without any substantial change in the structure of the ABO.sub.3
unit cell.
[0068] In order to provide good sinterability for firing of the
components, it is advantageous to repeat the step of micronization.
For this purpose, the powder is again mixed with deionized water
and finely milled. One should preferably aim for a grain size
distribution having roughly the following properties: d(0.5)=0.5
.mu.m and d(0.9).ltoreq.1 .mu.m. In the next step, a pressable
powder or granulate is produced from the milled suspension. This
can be carried out directly by spray-drying of a suspension mixed
with a binder, or--in the case of small amounts, for example,--by
drying the suspension and then manually adding a binder
component.
[0069] Shaping of the component is now carried out. Components are
preferably molded by means of dry pressing. For the production of
conversion modules, for example, rod-shaped or cylindrical
components are required. Before subsequent firing of the
components, pre-decarbonization is advantageous (using thermal
releasing agents). It has been found that firing of the components
is of great importance in configuring the thermoelectric properties
of the material described.
[0070] The measurements of sintering density were carried out on a
cylindrical component having a diameter of 11 mm and a height of
5.5 mm. The measurements of electrical conductivity and thermopower
were carried out on a cylindrical component having a diameter of 10
mm and a height of 1 mm. The measurements of thermal conductivity
were carried out on a cylindrical component having a diameter of 11
mm and a height of 1 mm.
Optimization of the Firing Process
Example: Ca.sub.0.95Dy.sub.0.05MnO.sub.3 and
Ca.sub.0.95Gd.sub.0.05MnO.sub.3
[0071] The optimized firing process developed is described below by
way of example for the materials Ca.sub.0.95Dy.sub.0.05MnO.sub.3
and Ca.sub.0.95Gd.sub.0.05O.sub.3. The method is not limited to
these materials, but was successfully used in producing all of the
tested formulations of a complex metal oxide.
[0072] A particularly high maximum firing temperature is used in
the method. However, the maximum firing temperature should be below
the melting temperature, as the component could otherwise melt and
be destroyed. The firing temperature is preferably just below the
melting temperature of the material used.
[0073] For example, the maximum firing temperature T.sub.max is
100.degree. C. below the melting temperature T.sub.S or above it,
i.e., T.sub.max.gtoreq.T.sub.S-100.degree. C. In an embodiment,
T.sub.max.gtoreq.T.sub.S-75.degree. C. is true, e.g.,
T.sub.max.gtoreq.T.sub.S-50.degree. C. The firing temperature is
preferably at least 10.degree. C. below the melting temperature,
i.e., T.sub.max.ltoreq.T.sub.S-10.degree. C. For example, the
firing temperature is in the range of 10.degree. C. to 50.degree.
C. below the melting temperature. For the materials tested here,
for example, the melting temperature is approx. 1400.degree. C.
[0074] In the method, a very long maintenance time at the maximum
temperature is preferred. In particular, the maintenance time is at
least 10 h. For example, the maintenance time is at least 15 h.
[0075] Sintering is preferably carried out in an atmosphere having
sufficient oxygen. For example, sintering is carried out in an air
atmosphere or an oxygen-enriched atmosphere.
[0076] Moreover, the method is characterized by a slow cooling
rate. In particular, a cooling rate of less than or equal to
1.degree. C./min is used in cooling from 1000.degree. C. to
600.degree. C.
[0077] Furthermore, in cooling from 1000.degree. C. to 600.degree.
C., an additional maintenance time of at least one hour is
preferably used.
[0078] The slow cooling rate and additional maintenance time allow
the most complete conversion from Mn.sup.3+ to Mn.sup.4+, so that
the compound obtained is as stoichiometric as possible and has
particularly favorable thermoelectric properties. For this purpose,
cooling below a specified temperature is required. On the other
hand, the rate of diffusion of the required oxygen in the ceramic
decreases with falling temperature. There is therefore an optimum
temperature for the maintenance time. In sintering in air and at
atmospheric pressure, this temperature is in the range of
700.degree. C. to 800.degree. C., e.g., 750.degree. C. Oxygen
uptake is accompanied by phase transitions, which can easily cause
the brittle ceramic to crack. A slow cooling rate in the range of
the phase transition and below makes it possible to produce a
ceramic having few or no cracks.
[0079] It has been found that by means of this method, it is
possible to find a process window within which favorable grain
growth with advantageous properties can be achieved without melting
of the ceramic. Moreover, it has been found that a material
manufactured in this manner is highly resistant to air and oxygen.
In particular, the material remains stable in air up to high
temperatures (.gtoreq.800.degree. C.).
[0080] The following table shows the electrical conductivity and
density of the fired ceramic for the two formulations at various
maximum firing temperatures.
TABLE-US-00001 Max. firing Electrical Density of temperature
conductivity ceramic Formulation (.degree. C.) (S/cm) (g/ml)
Ca.sub.0.95Dy.sub.0.05MnO.sub.3 1150 148 4.27 1250 304 4.66 1350
428 4.66 Ca.sub.0.95Gd.sub.0.05MnO.sub.3 1150 123 4.07 1250 285
4.62 1350 416 4.62
[0081] As can be seen from the table, at a maximum firing
temperature of T.sub.max=1150.degree. C., the electrical
conductivity .sigma. of the two formulations is below 150 S/cm. At
this firing temperature, the density of the ceramic is
.gamma.<4.3 g/ml for the two formulations. When the maximum
firing temperature is increased to T.sub.max=1250.degree. C.,
electrical conductivity increases sharply. The sintering density
also increases. On a further increase in the maximum firing
temperature to T.sub.max=1350.degree. C., the electrical
conductivity of the two formulations increases to a value of
.sigma.>400 S/cm. The density of the ceramic is .gamma.>4.6
g/ml.
[0082] FIG. 2 shows a graphical representation of electrical
conductivity G as a function of maximum firing temperature
T.sub.max for the two formulations. The electrical conductivity
shows virtually linear dependency on maximum firing
temperature.
[0083] FIG. 3 shows the microgram obtained in sintering as an
example for one of the working examples.
[0084] By means of the method used, taking a primary grain size of
0.5 .mu.m as a starting point, a stable and dense ceramic composed
of grains measuring 10 .mu.m in diameter can be produced. The
growth of the grains was therefore greater than one order of
magnitude. The favorable electrical conductivity can be attributed
to the large grain diameter, as in this case only minor dispersion
of the charge carriers takes place at the grain boundaries.
[0085] In the following, various materials and components
containing the materials are characterized. All of the materials or
components were produced by the above-described method. In
particular, the components of a complex metal oxide can be
determined by comparing their properties.
Example: Ca.sub.0.97La.sub.0.03MnO.sub.3
[0086] As a first example, a ceramic based on calcium manganese
oxide (calcium manganate) is tested in which Ca.sup.2+ has been
partially replaced by a suitable atom with a valence of 3+,
corresponding to donor doping in the A position. The ceramic is
represented by the formula Ca.sub.0.97La.sub.0.03MnO.sub.3.
Sintering was carried out at a maximum temperature of 1320.degree.
C.
[0087] The following properties in particular are relevant for
thermoelectric conversion. Characterization was conducted at room
temperature.
TABLE-US-00002 Sintering density .gamma. = 4.61 g/cm.sup.3
Electrical conductivity .sigma. = 258 S/cm Thermopower .alpha. =
-125 .mu.V/K Power factor (.sigma. .alpha..sup.2) PF = 4.06
10.sup.-4 W/(mK.sup.2) Thermal conductivity .kappa. = 3.89 W/(mK)
Figure of merit ZT = 0.033
[0088] For thermoelectric conversion, the dependency of the
properties on the surrounding temperature is of particular
interest. The ends of a thermoelectric component are at different
temperature levels. The amount of energy converted increases with
increasing temperature difference, provided that the figure of
merit does not decrease disproportionately with temperature.
[0089] FIG. 4 shows the temperature dependency of electrical
conductivity .sigma. for the Ca.sub.0.97La.sub.0.03MnO.sub.3
ceramic. The measurements were carried out in two components. The
components were produced under the same conditions. The virtually
identical measurement results demonstrate the favorable
reproducibility of component production and of the measurement
method.
[0090] Electrical conductivity .sigma. decreases with increasing
temperature. This reduction in conductivity with temperature is
also referred to as "metallic" behavior.
[0091] FIG. 5 shows the temperature dependency of the Seebeck
coefficient .alpha. for the two components. In this case, an
increase in the absolute value with increasing temperature can be
observed.
[0092] FIG. 6 shows the temperature dependency of thermal
conductivity K for one of the components. Thermal conductivity was
measured by means of a laser flash method. Thermal conductivity
decreases with increasing temperature.
[0093] Based on these measurements, the figure of merit ZT can be
derived by means of equation (1).
[0094] FIG. 7 shows the course of the figure of merit ZT, measured
in the two components of the Ca.sub.0.97La.sub.0.03MnO.sub.3
ceramic. The figure of merit reflects the efficiency of
thermoelectric conversion.
Example: Ca.sub.0.9Sr.sub.0.05Yb.sub.0.05MnO.sub.3
[0095] As a further example, a ceramic based on calcium manganate
was tested in which donor doping with Yb.sup.3+ was carried out
instead of donor doping with La.sup.3+. The doping content was also
increased from 3% to 5%. In this case, an increase in the number of
charge carriers and thus improved electrical conductivity is to be
expected. However, the number of charge carriers also affects the
result (See, e.g., "Heike's formula"). At a donor content of
y>50%, the conduction mechanism usually changes to hole
conduction, so the donor content should be less than 50%.
[0096] In addition, 5% of the Ca.sup.2+ atoms were replaced by
specific heavier Sr.sup.2+ atoms. With an unchanged unit cell of
the perovskite structure, this should make it possible to increase
the density of the material and reduce thermal conductivity.
[0097] The material is therefore represented by the formula
Ca.sub.0.9Sr.sub.0.05Yb.sub.0.05MnO.sub.3. The above-described
method was again used for production.
[0098] Characterization of the component at room temperature was
again conducted:
TABLE-US-00003 Sintering density .gamma. = 4.70 g/cm.sup.3
Electrical conductivity .sigma. = 399 S/cm Thermopower (Seebeck
coefficient) .alpha. = -101 .mu.V/K Power factor PF = 4.05
10.sup.-4 W/(mK.sup.2) Thermal conductivity .kappa. = 3.08 W/(mK)
Figure of merit ZT = 0.040
[0099] It can be derived from these values that the improved
electrical conductivity is compensated for by the reduced
thermopower, so that the power factor remains approximately the
same. An increase in sintering density of approx. 2% and a decrease
in thermal conductivity of approx. 20% can be observed, which
therefore also improves the figure of merit ZT by approx. 20%.
[0100] Overall, it was found that material modifications that
specifically lead to denser structures with reduced thermal
conductivity constitute an interesting alternative to material
changes that only alter the electronic properties of the oxide
ceramic.
Example: Ca.sub.0.85Sr.sub.0.10Dy.sub.0.05MnO.sub.3
[0101] As a further example, a ceramic based on calcium manganate
was tested in which even more Ca.sup.2+ atoms (10%) were
specifically replaced by heavier Sr.sup.2+ atoms. The donor doping
content was kept at 5%, but doping was carried out in this case
with Dy.sup.3+.
[0102] The material is therefore represented by the formula
Ca.sub.0.85Sr.sub.0.10Dy.sub.0.05MnO.sub.3. The above-described
method was again used for production.
[0103] At room temperature, the following properties are seen
compared to the preceding examples:
TABLE-US-00004 Thermal Comparison Sintering density conductivity
example Material (g/ml) (W/mK.sup.2) 1
Ca.sub.0.97La.sub.0.03MnO.sub.3 4.61 3.89 2
Ca.sub.0.90Sr.sub.0.05MnO.sub.3 4.70 3.08 3
Ca.sub.0.85Sr.sub.0.10Dy.sub.0.05MnO.sub.3 4.74 2.88
[0104] The Ca.sub.0.85Sr.sub.0.10Dy.sub.0.05MnO.sub.3 and
Ca.sub.0.9Sr.sub.0.05Yb.sub.0.05MnO.sub.3 ceramics thus show
increased sintering density and reduced thermal conductivity.
[0105] FIG. 8 shows the dependency of temperature on thermal
conductivity for the materials
[0106] Ca.sub.0.85Sr.sub.0.10Dy.sub.0.05MnO.sub.3 and
Ca.sub.0.9Sr.sub.0.05Yb.sub.0.05MnO.sub.3. It can be seen that
there is reduced thermal conductivity in the entire range of 300 to
1000 Kelvins.
[0107] The three examples show that the efficiency of
thermoelectric conversion can be improved by means of structures
having increased density and reduced thermal conductivity.
[0108] It would be expected that this effect could be further
increased by further or complete replacement of Ca.sup.2+ atoms by
specific heavier Sr.sup.2+ atoms. However, it was found that at a
content of over 20% Sr.sup.2+ atoms, there is an increasing change
in the unit cell of the perovskite, which thus alters the
electronic properties (conductivity, thermopower) in an unfavorable
manner. The changed structure of the unit cell can be seen, for
example, on the x-ray diffractogram (XRD).
[0109] It has been found that a further increase in efficiency can
be achieved by incorporating suitable atoms that are even heavier
than Sr.sup.2+. For example, Ba.sup.2+ and Pb.sup.2+ are suitable
for this purpose.
Example:
Ca.sub.0.85Sr.sub.0.10X.sub.0.05Mn.sub.1-zFe.sub.zO.sub.3(X.dbd.D-
y, Bi)
[0110] As a working example of a material based on CaMnO.sub.3
doped with Fe atoms that replace Mn atoms, a material is
characterized below that is represented by the formula
Ca.sub.0.85Sr.sub.0.10X.sub.0.05Mn.sub.1-zFe.sub.zO.sub.3, where X
is equal to Dy or Bi. A portion of the Mn atoms in the B positions
is therefore replaced by Fe atoms. The vast majority (>80%) of
the B positions are occupied by Mn atoms. For this reason, the
crystal structure and stability of the manganate compound that are
beneficial for thermoelectric conversion are largely retained.
[0111] FIG. 9 shows a comparison of the x-ray diffractograms for
the compounds
Ca.sub.0.85Sr.sub.0.10Bi.sub.0.05MnO.sub.3 and
Ca.sub.0.85Sr.sub.0.10Bi.sub.0.05Mn.sub.0.90Fe.sub.0.10O.sub.3.
[0112] A virtually identical reflex pattern can be seen, although
10% of the Mn atoms in the B position are replaced by Fe atoms.
This means that incorporation of the Fe atoms took place without
any substantial change in the structure of the ABO.sub.3 unit
cell.
[0113] In the following, the effect of the content of incorporated
Fe atoms is investigated in further detail. In particular, the
content z of Fe atoms in the material of formula
Ca.sub.0.85Sr.sub.0.10Dy.sub.0.05Mn.sub.1-zFe.sub.zO.sub.3 is
varied.
[0114] FIG. 10 shows the dependency of sintering density on the
content z of Fe atoms in this material. Fe contents of z=o %, 0.5%,
1%, 2.5%, 5% and 10% were tested. The compensation curve was
roughly estimated.
[0115] It can be seen from FIG. 10 that with an added amount of up
to 5% Fe, density is higher than the value for the Fe-free
compound. At 10% or above, density again decreases sharply. Because
of the increased density at up to 5% Fe, and because the Fe atoms
in the lattice are to be seen as defects for phonons, it can be
concluded that the thermal conductivity in this range is also below
the value for the Fe-free compound.
[0116] FIG. 11 shows the dependency of the Seebeck coefficient
.alpha. on the Fe content z of this material. Measurements were
conducted at room temperature. Fe contents of z=0%, 0.5%, 1%, 2.5%,
5% and 10% were again tested. The compensation curve was roughly
estimated.
[0117] Up to approx. 10% Fe content, thermopower is negative (the
material is of the "n-type"). Up to 5%, the absolute value of the
Seebeck coefficient increases. With addition of slightly more than
5% Fe, thermopower again decreases sharply.
[0118] The parameters of thermoelectric conversion can therefore be
optimized by means of the measurement values from FIGS. 10 and 11.
It has been found that a material having an Fe content in the range
of 0.0001 to 0.2 shows advantageous properties. At an Fe content of
z>0.2, electronic conductivity is extremely low.
Example:
Ca.sub.1-x-0.05Sr.sub.xDy.sub.0.05Mn.sub.1-zFe.sub.zO.sub.3
[0119] As a further working example, a material is characterized in
which the Sr content is increased from 10% to 20% compared to the
preceding working example. In particular, a material of the formula
Ca.sub.1-x-0.05Sr.sub.xDy.sub.0.05Mn.sub.1-zFe.sub.zO.sub.3 is
characterized. Again, a variation of the content z of Fe atoms is
tested.
[0120] FIG. 12 shows the dependency of sintering density .gamma. on
Fe content z for Sr content of x=10% and x=20%.
[0121] The incorporation of a greater number of "heavier" Sr atoms
increases the density of the ceramic produced and reduces its
thermal conductivity. However, it has been found that with an Sr
content of x>50%, the properties strongly resemble the more
unfavorable properties of SrMnO.sub.3.
[0122] Even with an Sr content of 20%, an additional positive
effect on sintering density is seen with Fe addition of up to
5%.
[0123] FIG. 13 shows the dependency of thermopower a on Fe content
z for an Sr content of x=10% and x=20%.
[0124] The resulting course is similar to that shown in the working
example of FIG. 11. Up to a content of approx. 10% of added Fe, the
thermopower is negative (the material is of the "n-type"). Up to
approx. 5% Fe content, the absolute value of thermopower increases
in an advantageous manner.
[0125] FIG. 14 shows a working example of a thermoelectric element
1, in particular a thermoelectric generator.
[0126] The generator has a so-called H structure. The generator is
configured as a module having a plurality of materials 2, 3 of
different types. The materials 2, 3 form the legs of the generator.
The first material 2 is of the n-type, and as described above, is
based on calcium manganese oxide. The second material 3 is of the
p-type. The two materials 2, 3 preferably have comparable figures
of merit. In this case, particularly favorable energy conversion
can be achieved overall.
[0127] For example, a sodium cobaltate based on the general formula
(Ca.sub.3-xNa.sub.x)Co.sub.4O.sub.9-.delta., where
0.1.ltoreq.x.ltoreq.2.9 and 0<.delta..ltoreq.2, and particularly
where 0.3.ltoreq.x.ltoreq.2.7 and 0<.delta..ltoreq.1, is used
for the second material 3.
[0128] The legs comprising the materials 2, 3 are thermally
parallel and electrically connected in series. Contacts 6 composed,
e.g., of an Ag paste are provided for electrical connection
purposes.
[0129] The generator has two electrical connections 4, 5. Thermal
contact elements 7, 8 are also present that simultaneously form
electrical insulators. Examples of compounds used for this purpose
include Al.sub.2O.sub.3, AlN and/or Si.sub.3N.sub.4. For example,
the materials 2, 3 are sintered together with the electrical
contacts 6 and the thermal contact elements 7, 8.
[0130] When there is a temperature difference between the two
contact elements 7, 8, a voltage referred to as thermopower is
generated between the electrical connections 4, 5.
[0131] In an alternative embodiment, a thermoelectric element, in
particular a thermoelectric generator, has only two legs composed
of different materials 2, 3.
* * * * *